1. Field of the Invention
The present invention relates to a backlight unit for flat panel displays, and more particularly, a direct-type backlight unit having improved heat-dissipating ability for flat panel displays.
2. Description of the Prior Art
Backlight units are known in the art. The backlight unit, which is a key element in the fabrication of liquid crystal displays, is widely used in digital cameras, PDAs, vehicle satellite navigation systems, computer monitors, flat panel TVs and so on. Typically, a backlight unit, which is generally installed underneath a display panel, comprises a light source (or multiple light sources) and a light diffusion means for providing users and consumers with diffused, ample, and comfortable backlighting. Light penetrates the overlying display panel and forms various images controlled by pixel electrodes densely arranged on the display panel. Backlight units are typically divided into two major categories: edge light type and direct-type, where the direct-type backlight unit can provide higher intensity of light and is thus more suited for large size display panels such as TV panels than the edge light type.
Referring to FIG. 1, a cross-sectional view of a conventional direct-type backlight unit is illustrated. The backlight unit 10 is installed underneath a display panel 12 and comprises a diffusion plate 16, a reflection plate 18, and multiple light tubes 14 arranged in parallel in a chamber 30 defined by the diffusion plate 16 and the reflection plate 18. The reflection plate 18 is used to reflect light generated by the light tubes 14 upward to the overlying diffusion plate 16 thereby increasing light use efficiency of the backlight unit. The diffusion plate 16 is used to diffuse light (or reflected light) by means of shielding, scattering, or refraction. The diffusion plate 16 is typically made of acrylic or polycarbonate (PC) materials having a thickness h1 of about 2 mm to 3 mm and a light transmission ratio of about 50% to 80%. According to a prior art method to generate diffused light, dispersed ink or light shielding tiny dots are printed onto a surface of the diffusion plate 16.
Still referring to FIG. 1, above the diffusion plate 16, a diffusing sheet 20 and an optic focusing film 22 are typically provided for enhancing backlighting. The diffusing sheet 20 (also referred to as a protection diffusing sheet) is made of PET or PC and has a higher light transmission ratio than that of the underlying diffusion plate 16, a lower haze, and a thickness of about 0.11 mm to 0.15 mm. It is known in the art that the number of the diffusing sheet 20 and the number of the optic focusing film 22 and arranging sequence of the two can be adjusted according to desired purposes.
However, the prior art direct-type backlight unit has a drawback that it has poor light use efficiency since light generated by the light sources is partially shielded by the ink dots printed at the lower surface of the diffusion plate 16. Besides, heat radiation is also a serious problem for prior art direct-type backlight units. In a practical case, heat accumulated in the chamber 30 reduces lifetime of the light tubes 14 and the high temperature on the diffusion plate 16 causes distortion of the optic focusing film 22, that, in turn, leads to poor light output performance. An approach to solving this heat buildup problem is using a heat-radiating fan installed at a periphery of the backlight module. Nevertheless, this introduces undesirable dust into the backlight unit casing and also consumes electricity. Consequently, there is a strong need to provide an improved direct-type backlight unit to elongate lifetime of light tubes, and, at the same time, thin down the backlight unit.